Plasma arc torches are widely used for the high temperature processing (e.g., cutting, welding, and marking) of metallic materials. A plasma arc torch generally includes a torch body, an electrode mounted within the body, an emissive insert disposed within a bore of the electrode, a nozzle with a central exit orifice, a shield, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. The gas can be non-reactive, e.g. nitrogen or argon, or reactive, e.g. oxygen or air.
In the process of plasma arc cutting or marking a metallic workpiece, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode) within a torch. When operating in this pilot arc mode the electrode can separate from the nozzle, forming an arc between these electrode and nozzle, e.g., as described in U.S. Pat. No. 4,791,268, the contents of which are incorporated herein by reference. The gas passing between the nozzle and the electrode is ionized to form a plasma, which then exits an exit orifice of the nozzle. The gas can be passed through a swirl ring to impart a tangential motion to the gas as it passes through the torch, thereby improving torch performance. When the torch is moved near a workpiece the arc contacts the workpiece and the current return path then transfers from the nozzle to the workpiece. Generally the torch is operated in this transferred plasma arc mode, which is characterized by the flow of ionized plasma gas from the electrode to the workpiece, with the current return path being from the workpiece back to the power supply. The plasma thus generated can be used to cut, weld, or mark workpieces.
In addition to blowback operation described above, alternative known techniques include blow forward technologies, in which the nozzle separates from a stationary nozzle. See, e.g., U.S. Pat. No. 5,994,663, the contents of which are incorporated herein by reference.
Dimensions of the torch are determined by the size and configuration of the consumables discussed above, e.g., the electrode, swirl ring, nozzle, and shield. Design of these consumables is highly technical and has a dramatic impact on torch life and performance. The electrode is generally surrounded by a swirl ring, a nozzle, and perhaps a shield. All of these components, and the way in which they are designed and combined, affect the overall torch dimensions, configuration, weight, cost, and other parameters.
Moreover, safety has always been a concern with plasma cutting torches because of the risk of electrical shock and burns. To minimize such risks, various safety systems have been employed to protect the torch operator. Some safety systems are designed to disengage the power supplied to the torch when components of the torch are missing or incorrectly assembled in the torch handle. Often, when operating a plasma cutting torch, consumable parts must be removed for inspection or replacement, and the torch components are disassembled and reassembled on site and immediately returned to service. This operation can at times be rushed, performed in poorly lit or dirty environments, or otherwise implemented incorrectly, leading to potentially dangerous errors in the reassembly and operation of the torch. The aforementioned safety systems typically include a sensing device that is engaged when a removable torch component is placed in its proper position in the torch handle. When functioning properly, the sensing device allows power from the power supply to supply the torch only when the removable component is placed in its proper position in the torch handle.
Existing safety systems, however, position sensitive safety system components near the operating end of the torch, which exposes these components to high temperatures generated at the torch tip. Existing safety systems also employ bulky handle designs to accommodate safety system components, but those bulky designs tend to obstruct or limit the operator's view of the workpiece. Each of these limitations can impede the operation of the torch and the efficient replacement of worn replaceable components, or lead to the failure of the safety system and, ultimately, to injury to the operator. For example, as shown in EP 0208134, a safety switch is placed near the end of a torch assembly, exposing the switch to the high temperatures associated with the torch. U.S. Pat. No. 6,096,993 shows an actuating element that is moved by an extension of a shroud, which requires a bulkier design around the torch component assembly to accommodate the actuating element.
In view of the limitations with above-described safety systems, it is desirable for a torch handle to have a safety system that positions sensitive safety components in the torch handle away from high temperature areas, and that does not add bulk to the torch assembly or obstruct the operator's view of the workpiece.